Modeling - symmetrizing part of a mesh
You can correct this shape manually, using two tools.

01) Scale tool
- correct the height of edge rings, so they're level.
- just scale them to 0 along the Z axis  



02) To Sphere Tool
- this tool will make a circle from any shape
- you can use it to correct the roundness of your edge rings
- select the edge ring and go Mesh > To Sphere (click anywhere to activate it)